Applied Surface Science, Vol.490, 420-429, 2019
Influence of additives and their molecular structure on the static and dynamic wetting of oil on steel at room temperature
The effect of additives, their type and structure on the wetting of oil has received little attention so far and is thus poorly understood. This is despite the fact that a number of additives are present in every lubricating oil. Here, we report on the influence of the addition of some simple organic friction modifiers, namely, fatty acids, amides, alcohols and amines, on the static and dynamic (i.e., the advancing and receding contact angles, and the contact-angle hysteresis) wetting of synthetic PAO oil on steel. The results show that the dynamic wetting parameters are much more sensitive to changes in the wetting than the static contact angle, and thus more appropriate for wetting studies of oils, where the most pronounced effect was observed with the receding contact angle. The additives in the oil reduced the wetting of the steel surface significantly. The above-mentioned receding contact angles increased up to 8.3 times compared to the oil without additives. The influence of the additive's molecular structure was investigated by varying four of their fundamental properties. The results indicate that the most polar fatty acid decreases the wettability by 98% more than the least polar amine, the longest chain length of 18C atoms, by 76% more than the shortest chain of 11C atoms, the additives with one polar head group, by 74% more than with two polar groups, and the saturated additive, by 39% more than the unsaturated additive.